One of the goals of plant genetic engineering is to produce plants with agronomically important characteristics or traits. Recent advances in genetic engineering have provided the requisite tools to transform plants to contain and express foreign genes. Particularly desirable traits or qualities of interest for plant genetic engineering include but are not limited to resistance to insects and other pests and disease-causing agents, tolerances to herbicides, enhanced stability, yield, yield stability, shelf-life, environmental tolerances, and nutritional enhancements. The technological advances in plant transformation and regeneration have enabled researchers to take pieces of DNA, such as a gene or genes from a heterologous source, or a native source, and incorporate the exogenous DNA into the plant's genome. The gene or gene(s) can then be expressed in the plant cell to exhibit the added characteristic(s) or trait(s). In one approach, expression of a novel gene that is not normally expressed in a particular plant or plant tissue confers a desired phenotypic effect(s).
Plant growth and development is controlled by a diverse array of phytohormones in response to endogenous signals and environmental cues. These molecules include auxins, gibberellins, ethylene, cytokinins, abscisic acid, brassinosteroids, oligosaccharides, jasmonates, salicylic acid, and polyamines. In recent years, the large-scale sequencing of plant genomes, together with extensive molecular, genetic and biochemical studies, has led to the elucidation of the biosynthetic pathways for cytokinin biosynthesis. This in turn has allowed for new approaches to crop improvement by manipulating the expression of genes regulating the plant hormone biosynthesis through novel combination of gene expression elements and structural genes. Prior work has shown through both forward and reverse genetic analyses that plant hormonal regulation is highly complex. This has made it difficult to develop transgenic plants with improved agronomic traits but without deleterious pleiotropic effects.
Isopentenyl transferase (IPT) is a gene that was first isolated from Agrobacterium tumefaciens T-DNA. It was shown that expression of IPT in transgenic plants resulted in elevated cytokinin accumulation, which was accompanied by morphological alterations of transgenic plants. Expression of IPT in an entire plant has phenotypes that are not advantageous, causing dwarfing, reduced leaf area, and thicker stems (Hlinkova, E., et al., Biologia Plantarum, Vol. 41: 25-37, 1998). Expression in entire plants also leads to stunting, loss of apical dominance, reduction in root initiation and growth, either acceleration or prolonged delayed senescence in leaves (depending on growth conditions), adventitious shoot formation from unwounded leaf veins and petioles, altered nutrient distribution, and abnormal tissue development in stems (Li, et al., Dev Biol 153: 386-395, 1992). When IPT is constitutively expressed at high level through either the CaMV 35S promoter or the native IPT promoter, the transgenic plants would develop severely retarded shooty morphology and would fail to root (Klee et al., Ann Rev Plant Phys 38:467-486, 1987). Whole plants transformed with IPT genes with weaker or controlled expression, such as the promoter of heat shock protein genes, also display the effects of cytokinin overproduction, such as uncontrolled axillary bud growth as a result of the loss of apical dominance, the development of small, rounded or curling leaves, and the retardation of root formation (Medford et al., Plant Cell 1:403-413, 1989; Schmulling et al., FEBS Letters 249:401-406, 1989; Smigocki, PNAS 85: 5131-5135, 1991; Hewelt et al., Plant J 6: 879-891, 1994; Smart et al., Plant Cell 647-656, 1991; Van Loven et al., J Exp Bot 44: 1671-1678, 1993; Faiss et al., Plant J 12: 401-425, 1997). It was also shown that low level of constitutive IPT expression, such as that directed by a heat shock promoter under non-inductive conditions, would be sufficient to induce abnormal plant growth and development (Medford et al., Plant Cell 1:403-413 1989; Smigocki, PNAS 85: 5131-5135, 1991). Remarkably, such deleterious pleiotrophs occurred even when some seemingly tissue or organ specific promoters were used to drive the IPT expression in plants. For example, when an auxin-inducible bidirectional promoter from the soybean SAUR gene was used to drive the expression of IPT in transgenic tobacco plants, the transgene-produced cytokinin was present in a tissue- and organ-specific manner. Yet the localized overproduction of cytokinins still resulted in a number of morphological and physiological off-types, including stunting, loss of apical dominance, reduction in root initiation and growth, abnormal leaf development, adventitious shoot formation from unwounded leaf veins and petioles, altered nutrient distribution, and abnormal tissue development in stems (Li, et al., Dev Biol 153: 386-395, 1992). When IPT was specifically expressed in developing seeds using a highly specific seed promoter, no seed size increase was observed (Roeckel et al., Transgenic Res 6:133-141, 1997). We show that, despite these and other teachings against expression of IPT in plants, the expression under certain cell cycle regulated promoters leads to advantageous phenotypes.